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There are three methods of structural design, i.e. working stress, limit state and ultimate load method of structural design. These design methods are used for reinforced concrete as well as steel structure design.Contents:
Methods of Structural Design
- Working stress method (WSM)
- Ultimate load method (ULM)
- Limit state method (LSM)
1. Working stress method (WSM)
This was the traditional method of design not only for reinforced concrete, but also for structural steel and timber design. The method basically assumes that the structural material behaves as a linear elastic manner, and that adequate safety can be ensured by suitably restricting the stresses in the material induced by the expected “working loads” on the structure. As the specified permissible stresses are kept well below the material strength, the assumption of linear elastic behavior is considered justifiable. The ratio of the strength of the material to the permissible stress is often referred to as the factor of safety. However, the main assumption linear elastic behavior and the tacit assumption that the stresses under working loads can be kept within the ‘permissible stresses’ are not found to be realistic. Many factors are responsible for this such as a long term effort of creep and shrinkage, the effects of stress concentrations, and other secondary effects. All such effects resulting significant local increases in a redistribution of the calculated stresses. The design usually results in relatively large sections of structural members, thereby resulting in better serviceability performance under the usual working loads.